The present invention relates to improvements in fluid cracking catalysts obtained by synthesizing high contents of zeolite Y in situ within macropores of silica-alumina microspheres composed of a mixture of calcined reactive kaolin clays, and preferably blending the high zeolite content microspheres with functional additives, such as activity adjusting microspheres, as described in U.S. Pat. No. 4,493,902. In particular, the invention provides an economically attractive means for increasing the zeolite content of the zeolitic microspheres, thereby increasing the activity of this component and permitting the use of larger amounts of the relatively less expensive functional additives and, preferably, resulting in cracking catalyst blends having improved selectively when used to crack petroleum feedstocks to produce transportation fuels.
U.K. No. 1,271,450 and 1,342,977 (e.g., EXAMPLES 2 and 4 of the latter) illustrate the preparation of cracking catalysts particles containing synthetic faujasite in the 50-200 micron size range by spray drying an aqueous slurry of raw kaolin, calcining the spray dried particles at 1300.degree. F. (or at 1000.degree. F. and then at 1300.degree. F. ) to convert the kaolin to metakaolin, mixing the particles with a sodium silicate-sodium hydroxide solution, adding seeds and refluxing to crystallize the zeolite. U.S. Pat. No. 3,377,006 teaches the preparation of pure zeolite Y by reaction of finely divided metakaolin with sodium silicate in the presence of seeds. Kaolin calcined through the exotherm is not utilized in practice of any of these prior art processes and the significant benefits we have observed that result from including this form of calcined clay in the reaction mixture would not be realized by the prior art.
The following are illustrative of patents that disclose the use of kaolin calcined through the exotherm, alone or with metakaolin, in zeolite synthesis, including in situ zeolite synthesis by reaction of a calcined clay with sodium hydroxide solution; generally the processes result in relatively low levels, e.g., 20-30%, of sodium zeolite Y.
U.S. Pat. No. 3,367,886 PA1 U.S. Pat. No. 3,367,887 PA1 U.S. Pat. No. 3,506,594 PA1 U.S. Pat. No. 3,647,718 PA1 U.S. Pat. No. 3,657,154 PA1 U.S. Pat. No. 3,663,165 PA1 U.S. Pat. No. 3,932,268 PA1 Reagents (All ACS Reagent Grade)
U.S. Pat. No. 4,235,753 discloses a process for crystallizing zeolite Y in microspheres by hydrothermal reaction between microspheres composed of metakaolin and separate microspheres composed of kaolin calcined through the exotherm by reaction with sodium hydroxide solution in the presence of seeds. Illustrative examples indicate the crystallized products contained a maximum of 30% zeolite, although the patent mentions crystallized products containing 2 to 75%, and most preferably 10-50% zeolite.
U.S. Pat. No. 4,493,902, the teaching of which are incorporated herein by cross-reference, discloses novel fluid cracking catalysts comprising attrition-resistant, high zeolite content, catalytically active microspheres containing more than about 40%, preferably 50.degree.-70% by weight Y faujasite and methods for making such catalysts by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two different forms of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. In a preferred embodiments, the microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution which is heated, preferably until the maximum obtainable amount of Y faujasite is crystallized in the microspheres.
In practice of the '902 technology, the porous microspheres in which the zeolite is crystallized are preferably prepared by forming an aqueous slurry of powdered raw (hydrated) kaolin clay (A1.sub.2 O.sub.3 :2SiO.sub.2 : 2H.sub.2 O)) and powdered calcined kaolin clay that has undergone the exotherm together with a minor amount of sodium silicate which acts as fluidizing agent for the slurry that is charged to a spray dryer to form microspheres and then functions to provide physical integrity to the components of the spray dried microspheres. The spray dried microspheres containing a mixture of hydrated kaolin clay and kaolin calcined to undergo the exotherm are then calcined under controlled conditions, less severe than those required to cause kaolin to undergo the exotherm, in order to dehydrate the hydrated kaolin clay portion of the microspheres and to effect its conversion into metakaolin, this resulting in microspheres containing the desired mixture of metakaolin, kaolin calcined to undergo the exotherm and sodium silicate binder In illustrative examples of the '902 patent, about equal weights of hydrated clay and spinel are present in the spray dryer feed and the resulting calcined microspheres contain somewhat more clay that has undergone the exotherm than metakaolin. The '902 patent teaches (col. 8) that the calcined microspheres comprise about 30-60% by weight metakaolin and about 40-70% by weight kaolin characterized through its characteristic exotherm It is to be noted that no metakaolin is present in the spray dryer feed used in the preferred manufacturing process described in the '902 patent. A less preferred method described in the patent at column 6, involves spray drying a slurry containing a mixture of kaolin clay previously calcined to metakaolin condition and kaolin calcined to undergo the exotherm but without including any hydrated kaolin in the slurry, thus providing microspheres containing both metakaolin and kaolin calcined to undergo the exotherm directly, without calcining to convert hydrated kaolin to metakaolin However, the patent teaches that less attrition zeolitized microspheres are produced by this approach.
In carrying out the invention described in the '902 patent, the microspheres composed of kaolin calcined to undergo the exotherm and metakaolin are reacted with a caustic enriched sodium silicate solution in the presence of a crystallization initiator (seeds) to convert silica and alumina in the microspheres into synthetic sodium faujasite (zeolite Y). The microspheres are separated from the sodium silicate mother liquor, ion-exchanged with rare earth, ammonium ions or both to form rare earth or various known stabilized forms of catalysts. The technology of the '902 patent provides means for achieving a desirable and unique combination of high zeolite content associated with high activity, good selectivity and thermal stability, as well as hardness (attrition-resistance).
EPA No. 0,194,101, published Sep. 10, 1986 which claims priority from U.S. Ser. No. 707,635, 707,362, and 750,813, all now abandoned, describes variations of the ion-exchange treatment applied to the sodium form high zeolite content microspheres of the '902 patent to provide so-called "octane" catalysts, the zeolite component of which is characterized by having a low sodium content, reduced unit cell size and the absence of rare earth or the permissible presence of minimal amounts of rare earth These known variations of zeolite Y faujasite are frequently referred to as stabilized and/or ultrastabilized zeolite Y. Hereinafter the various stabilized forms of zeolite Y, e.g., calcined H-Y, H-Re-Y, will be called ultrastabilized Y which now has a broader meaning than the original term which was limited to zeolite Y having unit cell size below 24.45 Angstrom units.
As described in the above-cited '902 patent and EPA '101, the high zeolite content, high activity microspheres are adapted to be blended with lower activity functional additives such as microspheres composed of calcined kaolin clay and/or microspheres containing a vanadium immobilizing agent, a preferred form of the latter being the magnesia-enriched calcined kaolin clay microspheres described in EPA 06/937,457, the teachings of which are incorporated herein by cross-reference. In some cases blends may include other catalytic microspheres which function to adjust activity, selectivity or both.
The zeolite content of the crystallized microspheres is determined by X-ray diffraction from the zeolite which is best performed on the sodium form crystallized microspheres Conventional chemical analytical techniques are not deemed to be applicable to the determination of the zeolite content of materials in which the zeolite is crystallized in situ in a silica-alumina matrix which cannot be readily physically or chemically isolated. In practice, it has been found that the apparent amount of zeolite crystallized from any given formulation using the '902 technology can vary, depending on the history of raw material, processing conditions and proportions and concentrations of reagents. The zeolite content (sodium form) of crystallized microspheres range from 44% to 72% in illustrative examples of the '902 patent. Commercial production and laboratory preparations typically result in the crystallization of a maximum of about 55-60% zeolite (sodium form). Since at least a substantial proportion of the zeolite grows in macropores of the precursor porous microspheres, it might be expected that simply increasing macroporosity of the precursor microspheres would result in the generation of higher levels of zeolite because more space would be available in which to grow zeolite crystals.
Surprisingly, merely providing more room for crystal growth by increasing macroporosity will not achieve this result.
The aforementioned technology has met widespread commercial success Because of the availability of high zeolite content microspheres which are also attrition-resistant, custom designed catalysts are now available to oil refineries with specific performance goals, such as improved activity and/or selectivity without incurring costly mechanical redesigns. A significant portion of the FCC catalysts presently supplied to domestic and foreign oil refiners is based on this technology. Refineries whose FCC units are limited by the maximum tolerable regenerator temperature or by air blower capacity seek selectivity improvements resulting in reductions in coke make while the gas compressor limitations make catalysts that reduce gas make highly desirable Seemingly a small reduction in coke can represent a significant economic benefit to the operation of an FCC unit with air blower or regenerator temperature limitations
Improvements in cracking activity and gasoline selectivity of cracking catalysts do not necessarily hand in hand. Thus, a cracking catalyst can have outstandingly high cracking activity, but if the activity results in a high level of conversion to coke and/or gas at the expense of gasoline the catalyst will have limited utility. Catalytic cracking activity in present day FCC catalysts is attributable to both the zeolite and nonzeolite (e.g, matrix) components. Zeolite cracking tends to be gasoline selective. Matrix cracking tends to be less gasoline selective. After appropriate ion-exchange treatments with rare earth cations, high zeolite content microspheres produced by the in situ procedure described in the '902 patent are both highly active and highly gasoline selective. As zeolite content of these unblended microspheres is increased, both activity and selectivity tend to increase. This may be explained by the decrease in matrix content with increase in zeolite content and the decreasingly prominent role of nonselective matrix cracking. Thus, increases in the zeolite content of the high zeolite content microspheres are highly desirable.
Octane catalysts present a major drive for the supply of zeolite catalysts of increased activity without detriment to selectivity and hardness An increasingly large proportion of the FCC catalyst being used at present is represented by so-called "octane catalysts" which are formulated to boost the octane of the FCC gasoline fraction of the cracked oil feedstock. Generally, octane catalysts are of the ultrastable zeolite Y type and are prepared by post treating zeolite Y, synthesized in sodium form, to exchange sodium with ammonium and/or hydrogen followed by a thermal treatment that reduces unit cell size of the zeolite, resulting in so-called ultrastabilized zeolite. Frequently, multiple exchanges and calcinations are used. Metal ions such as rare earth ions which contribute to hydrogen transfer reactions are not present or are present in limited amounts.
Known treatments used to provide octane catalysts by providing ultrastabilized zeolite invariably tend to result in catalysts that are less active than and less gasoline selective than similar catalysts ion-exchanged with rare earth. It has been found that this is generally also true of stabilized octane catalysts prepared by the '902 technology. Octane version of these catalysts are both less active than and less gasoline selective than the high rare earth content counterparts of the '902 patent. It has been found that laboratory based data in EPA No. 0,194,101 has been found that priority documents are not consistently reproducible.
Consequently, the desirability of increasing the activity of the high zeolite content microspheres of the '902 patent is especially significant when applying downstream processing to prepare ultrastabilized octane catalysts. Because increases in zeolite content is generally associated with increases in cracking activity, as discussed above, the desirability of increasing the levels of zeolite in higher zeolite content microspheres without significantly impairing attrition-resistance or selectivity would represent a significant technological advance.
In view of the commercial importance of FCC catalyst blends based on high zeolite content microspheres, especially but not limited to octane catalysts, there has been a continuing search for means to produce high zeolite content microspheres having increased cracking activity without sacrifice in selectivity and thermal stability, and preferably having improved activity and selectivity. This present invention is a result of these searches.